1. Technical Field
This invention generally relates to an apparatus and method for vaporizing liquid natural gas for use as an engine fuel, and more particularly to an apparatus and method for tracking vaporized fuel temperature with pressurized combustion air temperature throughout a range of ambient air temperature and operating conditions.
2. Background Art
The railroad industry, like the automotive and truck industries, is faced with the dual challenges of improving operating cost efficiency, and at the same time reducing pollutant emissions, including nitrogen oxides (NO.sub.x), hydrocarbons, and carbon particulates. The challenge is aggravated by the fact that with most engine fuels, particularly diesel no. 2, these goals are almost mutually exclusive. Reducing emissions usually results in decreased operating efficiency.
Using liquified natural gas, or liquified methane, stored at cryogenic temperatures provides an attractive alternative to diesel fuel since both operating cost savings and pollutant emissions reductions can be achieved. Two particularly attractive fuels are methane, CH.sub.4, and various natural gas mixtures of methane, CH.sub.4, ethane C.sub.2 H.sub.6, and propane C.sub.3 H.sub.8.
However, there are some inherent problems, at least in the locomotive industry, with regards to utilizing liquid natural gas (LNG) as a fuel for a railroad locomotive. First, in order to carry a sufficient quantity of fuel, the methane or natural gas must be liquified and stored at cryogenic temperatures, in specially insulated pressure vessels. This fuel must then later be reheated, or vaporized, in order to be usable. Another problem is that with the use of LNG in a spark ignited otto cycle engine, although the combustible air/fuel ratio range is relatively wide, the air to fuel mass ratio required for stable operation is relatively narrow. In fact, at low engine load conditions, or at idle engine conditions, the air/fuel mixture ratio required for stable combustion is narrowly banded about the stoichiometric point. Engine manufacturers rely on sensitive engine air/fuel ratio control to meet difficult criteria, including strict emission standards.
The stoichiometric equation for the combustion of methane is represented by the balanced equation in which all of the carbon in the fuel is converted to carbon dioxide and all of the hydrogen to water. In the case of methane, the equation is CH.sub.4 +20.sub.2 =CO.sub.2 +2H.sub.2 O. Since oxygen is only one component of air, a more complete stoichiometric equation for combustion of methane, in air, is CH.sub.4 +2(0.sub.2 +3.3773N.sub.2)=CO.sub.2 +2H.sub.2 O+6,546N.sub.2. Substituting molecular weights for oxygen, average atmospheric nitrogen, atomic carbon and atomic hydrogen, results in an air to fuel mass ratio of 13.77 moles of air to each mole of fuel. This is a mass ratio, not a volumetric ratio. Thus, to maintain stoichiometric operating conditions, the challenge is to maintain the proper air to fuel mass ratio over a wide variety of operating conditions.
There are a number of different ways of adding the vaporized LNG to the charged air. They can be broken down into two broad categories, the first being the use of a carburetor to mix vaporized LNG with the charged air prior to its introduction into the combustion chambers, and the second, a direct injection of fuel into the combustion chambers of each cylinder. Carburetors are only used with spark ignited engines. Direct injection, or fuel injection, are used both in spark ignited and diesel engines.
In a carburetor, the charged air flows through a converging, diverging nozzle called the venturi. A pressure differential is set up between the carburetor inlet and the throat of the nozzle and is used to meter the appropriate flow of vaporized LNG for the given air flow. This is a volume flow rate metering system, not a mass flow rate metering system. Various design parameters for the carburetor, throttling devices, and pressure regulating devices for the vaporized LNG can be used to adjust the volumetric metering of the carburetor such that the air to vaporized LNG ratio of the volumetric metering system matches the air to fuel mass ratio for design combustion conditions for any given engine operating range for fixed ambient air and atmospheric pressure conditions. Changes in ambient air temperature or pressure, supercharger aftercooler temperature, or changes in fuel temperature or pressure will cause the volumetric ratio of the carburetion system to diverge from the air to fuel stoichiometric mass ratio required for proper combustion for low load conditions.
Changes in atmospheric pressure are primarily altitude dependent. These changes can be compensated for by either restricting operation of a given carbureted LNG engine to a given altitude range, or by some sort of pressure regulation for controlling the supply of vaporized LNG to the carburetor. These pressure changes can be significant, for example, atmospheric pressure at 5,000 feet above sea level is approximately 17% less than atmospheric pressure at mean sea level.
Ambient temperature variations, especially winter to summer, can produce changes of comparable magnitude to altitude changes. There are numerous locations throughout North America where ambient temperature variations can vary from lows of -40.degree. F. in winter to highs exceeding 110.degree. F. in summer. The air density changes resulting from ambient air temperature variations are not compensated for in the volumetric metering of the carburetor in a natural gas engine. This is particularly true when the engine is lightly loaded and idling where the band width for the mass ratio of air to fuel is a narrow band near stoichiometric. In cold weather, at idling conditions, the cold, dense mass of air passing through the volumetric carburetor can increase the volumetric air to fuel ratio to where the mixture is too lean to burn. Conversely, with idling conditions and very hot ambient air, the mass of the air passing through the volumetric carburetor will be much lower, and can result in an air/fuel mixture that is too rich for combustion.
The amount of power generated in an engine of given size is, in a very basic sense, dependent upon the amount of air and fuel mixed together and therefor the amount of fuel combusted within the engine. For that reason, exhaust gas turbochargers or mechanically powered superchargers are incorporated in most of the engine configurations for locomotives. Turbochargers or superchargers compress the combustion air before it is introduced into the cylinder combustion chambers to increase output horsepower. Unfortunately, compressing the combustion air increases its temperature and results in higher NO.sub.x emissions and premature detonation in spark ignited engines. As a result, locomotives utilizing turbochargers or superchargers incorporate aftercoolers to extract some of the heat added to the charged combustion air during the compression process.
Turbochargers and superchargers add significant amounts of heat to the combustion air during the compression process. At most operating speeds and under most ambient air conditions, the turbocharging or supercharging process will add enough heat to the compressed combustion air to raise its temperature above that desired for efficient steady state operation of the natural gas engine. As a result, under most operating conditions, the compressed air must be cooled in an aftercooler to bring its temperature back down to the desired set point intake air temperature. This aftercooling of combustion air to reduce its temperature to a steady state set point will compensate for most changes in air density caused by ambient air temperatures, particularly at high power and relatively high ambient air conditions. Thus, under warm, steady state, high power operation, the volumetric air to fuel flow ratio of the carburetor can be regulated to the air to fuel ratio required for proper combustion.
Under idle or low power operating conditions, or during extreme cold ambient air conditions, the turbocharging or supercharging process may not add enough heat to bring the compressed combustion air up to the design set point temperature. It is during these conditions that the volumetric air to fuel ratio established by the carburetor may not match, or track, with the required air to fuel mass ratio.
Accordingly, it is an object of this invention to provide a method and apparatus for adjusting the density of the vaporized LNG during engine warm up, idling, low power, and extremely cold ambient air conditions, so as to more closely match the volumetric air to fuel ratio of the carburetor to the stoichiometric mass air to fuel ratio required for efficient combustion. It is another object of the present invention to maintain thermal equilibrium between the vaporized LNG and the combustion air during overall steady state, high power, and high ambient air operating conditions.
It is an object of the present invention to provide for automatically adjusting the temperature, and thus the mass density, of vaporized LNG to compensate for mass density changes in combustion air caused by changes in ambient air temperature. And finally, it is an object of this invention to utilize the combination of engine jacket water heat and aftercooler heat to best accomplish these adjustments.